Bug 1245013 - Move CMFLAGS/CMMFLAGS from config.mk to configure. r=mshal
We don't really care to set those in js/src/configure because the JS
engine doesn't use ObjC. We also don't care to preserve the += behavior
because there were no AC_SUBST in the first place, so it's unlikely
anyone has an override in their mozconfig and expects it to work.